Mourners carry the coffin of a child at the funeral procession for those killed in an airstrike on a bus in Yemen.
Photograph: Mohammed Hamoud/Getty Images

The bombing of a bus full of schoolchildren last week was just one of more than 50 airstrikes against civilian vehicles by the Saudi-led coalition in Yemen so far this year, according to new data.

The data also shows that the monitoring body set up in Riyadh purportedly to investigate incidents of civilian casualties has supported the Saudi military version of events in almost every case.

The Joint Incidents Assessment Team (JIAT) has not issued comprehensive statistics but has instead issued periodic press statements. And according to an analysis by Human Rights Watch (HRW), out of 75 incidents where civilian casualties were reported, JIAT has admitted Saudi rules of engagement may have been broken in only two.

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In 10 more cases, JIAT has conceded that civilians may have been killed in error and said that compensation would be paid, but human rights advocates said there was so far no sign of any payments being made.

“None of the victims’ families we reached out to said they had been contacted,” said Kristine Bekerle, a HRW researcher, noting that the cases where compensation had been promised were up to two years old.

The 9 August airstrike obliterated a bus carrying schoolboys on a field trip in northern Yemen, leaving 40 children and 11 adults dead. The Saudi government told the UN security council that the strike would be investigated – but described it as “a legitimate military action”, saying it had targeted “Houthi leaders who were responsible for recruiting and training young children, and then sending them to battlefields”.

The schoolboys had been on the way to a cemetery where rebels were buried but survivors told journalists that was because it was one of the very few green spaces left in the whole of the northern Saada governorate. Parks and gardens have been destroyed in the relentless fighting.

“They came to the hospital in cars and ambulances. Dozens of children with an array of grisly wounds,” Marta Rivas Blanco, an ICRC nurse at the Al Talh hospital where the victims were taken, wrote in an account of the day for the Guardian. “Some were screaming, some were scared, many went straight to the morgue.”

A Yemeni child receives medical treatment after he was injured by the airstrike on the school bus. Photograph: Mohammed Hamoud/Getty Images

Statistics collated by an independent monitoring group, the Yemen Data Project, suggest that the targeting of the schoolbus was part of a wider pattern. According to its records, there have been 55 airstrikes against civilian vehicles and buses in the first seven months of this year – a higher rate than in 2017.

The group’s analysis of over 18,000 strikes from March 2015 to April 2016 found that almost a third (31%) of the targets were non-military – civilians or civilian infrastructure – 36% were military, while another third were classified as having an unknown target.

The coalition, whose other principal member is the United Arab Emirates, is under little international pressure to rein in its three-year bombing campaign and its stranglehold of trade routes.

Its efforts to isolate rebel-held areas culminated in a June offensive on the port city of Hodeidah, the distribution point for 80% of the food, water and fuel supplies for 8 million Yemenis in severe need.

The US defence secretary, James Mattis, said he had dispatched a three-star US general to Riyadh to look into the bus bombing. Meanwhile, a congressional defence spending bill signed by Donald Trump on Monday includes a clause requiring the secretary of state, Mike Pompeo, to certify that Saudi Arabia and the UAE are taking steps to prevent civilian deaths. Without such certification, the US would stop refuelling Saudi warplanes involved in the bombing campaign.

But the attack on the schoolbus was not mentioned in a state department account of a subsequent phone conversation between Pompeo and the Saudi crown prince, Mohammed bin Salman. Trump has yet to refer to it at all.

In the past, the US and UK governments – the coalition’s main arms suppliers – have pointed to the establishment of JIAT as evidence of an effort to reduce civilian casualties, and maintained their weapons sales. Saudi Arabia is the biggest single buyer of arms from both countries, accounting for about half the total UK and US sales, worth billions of dollars.

In May, Reuters reported that the Trump administration had asked Congress to review the sale of 120,000 Raytheon guided missiles to Saudi Arabia and UAE.

CNN reported on 17 August that fragments from the site of the schoolbus bombing were from a Lockheed Martin munition.

The US, UK and France, the third biggest arms exporter to the Middle East, have shielded their Gulf allies from serious diplomatic pressure at the security council.When Sweden, the Netherlands, Poland, Peru and Bolivia called for an emergency security council meeting on Friday to discuss the bus bombing, they were not joined by any of the five permanent members, although the UK, which currently holds the council presidency, scheduled the meeting.

Yemeni children raise signs and chant slogans during a demonstration against the airstrike on the school bus. Photograph: Mohammed Huwais/AFP/Getty Images

Kuwait, a member of the Saudi-led coalition in Yemen, used its seat on the council to block any strong language. There was no joint press statement at the end of the session, but merely “press elements”, a weaker expression of opinion, which watered down the call by the UN secretary general, António Guterres, for an “independent and prompt” investigation. The inquiry should be “credible and transparent”, the council members said, leaving open the possibility that the coalition members could investigate themselves.

The UK’s response to the bus attack came in the form of a tweet posted by Alistair Burt, the Foreign Office minister for the Middle East, who said he was “deeply concerned” about reports of children’s deaths, but did not mention the attack was carried out by the Saudi-led coalition.

Andrew Smith of the Campaign Against Arms Trade said “the complicit silence from No 10 is a clear case of arms company profits being put above human rights and Yemeni lives”.

Critics of US and UK policy argue that by continuing to arm the coalition while accepting whitewashed Saudi accounts of the civilian deaths, the two countries risk becoming legally complicit in war crimes.

“When the Saudi-led coalition’s investigative process basically says they are not responsible for ninety-something percent of these airstrikes hitting civilians, it calls into question the legitimacy of that process,” said Ted Lieu, a Democratic congressman from California.

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This week Lieu wrote to the acting inspector general at the Pentagon calling on him to launch an investigation over whether US service members providing support for coalition air operations, or US officials selling arms to Saudi Arabia, “could be responsible for aiding and abetting war crimes”.

The congressman told the Guardian: “I believe both the US and United Kingdom and other governments providing support to the Saudi-led coalition could be liable for war crimes.”

Fabian Hamilton, the UK’s shadow minister for peace and disarmament, said: “As a permanent member of the UN security council, the UK government’s response to this horrific attack has been abysmal.

“Failing to take action now means the government is presiding over the worst man-made humanitarian disaster of the 21st century,” Hamilton said. “Britain is currently playing a part in arming the Saudi-led coalition to the teeth, as it continues to violate international law, targeting civilians and sources of international aid.”

• This article was amended on 19 August 2018 to change attribution of fragments found at the site of the schoolbus bombing from a Raytheon munition to a Lockheed Martin munition.